Lightning Rods:

[rattus]

Do lightning rods bleed charge from a building and therefore prevent a strike, or do they actually increase the likelihood of a strike?

And do streamers indicate a flow of charge.

Seems that what I was taught 50 years ago may not be true anymore.

I grew up on a cotton farm in Texas, and once a little thunder storm came up. We all ran for the back porch, and the family helping us grabbed their kids' ears. We asked why? They replied, "To keep the lightning away!" Don't laugh, it worked!

 

[bob]

Re: Lightning Rods:

Lightning is a very complex phenomenon. They do not "bleed a charge" and prevent a strike. They do provide a low impedance path for the current to flow.
Check this location for more information.
http://wvlightning.com/protection.html

 

[physis]

Re: Lightning Rods:

I'm by no means one of the experts on lightning but my understanding is that the reason you would use a lightning rod is because you are already at higher elevation and at greater risk so you attempt to put a potential strike at an intentional location.

Edit: As far as increasing the likelyhood of a strike, again only the way I understand it, if there's gonna be a strike somewhere that's just how it's gonna be. But lighting is attracted to the easiest path for itself.

[ November 28, 2004, 08:18 PM: Message edited by: physis ]

 

[bphgravity]

Re: Lightning Rods:

My understanding of lightning is different. Lightning doesn't seek "ground" any more then our typical electrical systems. Lightning is the result of a charge separation between two points. This can be cloud to ground, ground to cloud, or cloud to cloud.

My belief is that a lightning protecton systems facilitate bringing the charges together, and not assist lightning to earth. The faster the charge is neutralized, the faster the damaging effects go away. So, by placing an air terminal at the highest point of the building, you bring the charge of the earth as close to the sky charge as possible an thus shortening the time the lightning event can take place. Basically, we are trying to bond the earth to the sky.

There is evidence of this. Studies have shown that lightning doesn't dive into the Earth and go away. It actually dances on the surface with fantastic arrays and swirling fields. One study I read claimed that lightning never made it more than a foot or two below the surface but spread out along the surface more than 40 feet in all directions.

Unfortunately, I don't believe there is any more evidence that lighting protection systems will prevent damage from lightning then will a family grabbing their ears on the porch during a storm. As funny as that sounds.

Personally, I feel the best lightning protection system is with TVSS and surge arrestor equipment placed on the service.

[ November 28, 2004, 09:41 PM: Message edited by: bphgravity ]

 

[physis]

Re: Lightning Rods:

I agree with most of what you said Bryan but there are two things I would put differently.

My belief is that a lightning protecton systems facilitate bringing the charges together, and not assist lightning to earth.

I think when we try to protect buildings from lighting, it's the lightning that's seeking earth we're attempting to deal with. Most of the time I don't think cloud to cloud lightning affects our disigns.

Basically, we are trying to bond the earth to the sky.

If that were possible then you could bleed off the charge. But you won't have any kind of access to the charges floating around until the voltage or potential is high enough to ionize the atmosphere. Then the lightning will do the bonding. All you can do is offer it a path you'd hope it would use.

 

[hurk27]

Re: Lightning Rods:

Lightning protection is a very complex system and require a very high amount of knowledge to install correctly.

The primary function is to safely dissipate a lightning charge to Earth without doing damage or causing injury to what it is protecting.

This alone requires the understanding of what lightning is, what it is trying to do, and the paths it will take when it does it.

There are many forms of lightning and each will have its own agenda as to the path it takes to seek its source. But the lightning we are trying to protect from it of the type that in all instances will be seeking Earth, and with that then there are two forms of this type:
1. Ground to cloud (negative)
2. Cloud to ground (positive)
The latter being the most destructive of the two.

Over the years there have been many "Old Wifes Tale's" In that some had scientific credences while others didn't, and in some cases even got people killed. This is why Knowledge is of most importance when dealing with a very dangerous installation as lightning protection is. Or one could end up on the wrong end of a law suit.

If you like to read then go to this place:

National Lightning Safety Institute (NLSI)

It one of the most largest amount of studys that have been done on lightning and effects.

 

[wirenut1980]

Re: Lightning Rods:

I think the idea is to picture a big circus tent covering whatever is is you want protected with the "tops" being the top points of your lightning protection system and the "sags" of the tent cannot touch the structure to be protected. Whether this is accomplished with an integral system, catenary system, or a mast system, I believe that is the idea. The lightning is supposed to strike the LPS instead of the structure being protected.

Something else to picture is the "rolling ball." The radius of the ball depends on the type of facility being protected, but usually 100 feet, or 150 feet radius. This distance comes from the theory that lightning changes the direction that it travels in 150 foot increments. So the idea is if you can roll a 150 foot imaginary ball over your building without it touching the building (it rolls on the LPS components), then you facility should be protected assuming the LPS is installed correctly. And in the previous example, the ball would be rolling on top of the circus tent.

With all the advances in technology it amazes me how much is yet to be discovered about lightning.

 

[john m. caloggero]

Re: Lightning Rods:

Lightning rods place in certain locations on buildings or structures serve two very specific function. (1)The lighting rod (air terminal) provides an attachment point for the lightning bolt to connect to instead of the building structure. (2)The rod also is a location that develops streamers or leaders upward that will intercept a downward leader from the charged cloud.

 

[saadt]

Re: Lightning Rods:

Wirenut1980 said:

The radius of the ball depends on the type of facility being protected, but usually 100 feet, or 150 feet radius. This distance comes from the theory that lightning changes the direction that it travels in 150 foot increments.

I?d like to complement the above information by saying that, the striking distance can be evaluated by the Golde (1977) formula :
ds= 32,8*exp(0,65*ln(I))
Where ds is in feet and I is the peak current of lightning in kA.
Thus, if I= 10,37 kA then ds=150ft

For more details, see:
The Lightning Discharge
by Martin A. Uman
The Dover (2001) edition
pg. 100

 

[hurk27]

Re: Lightning Rods:

So very true John. It has been questioned if installing a lightning protection system will just cause more strikes to a building that would normally not have been. While this might be a good argument not to install a system, it is also wrong thinking if the building is in a location that might be at a higher than normal strike risk. Like on a hill or it has the highest peak in the valley. This reminds me of a very good saying by one of my professors at University of Gainesville said:
It would always be much safer to have 10,000 directed lightning strikes to Earth with a properly installed system, than to have only one undirected strike to a structure. Or something like that.

[ November 29, 2004, 09:58 PM: Message edited by: hurk27 ]

 

[pierre]

Re: Lightning Rods:

From my reading and understanding of lightning, I think most are confused, because ligntning does not act quite like the electrical current we produce and harness. Lightning is a static discharge between two dissimilar charges (+ and -). That is why we see cloud to cloud, and so many trees struck by lightning. The notion that lightning is seeking earth is mostly not true.

As has been mentioned, properly installing a lightning protection system takes a different learned skill. My understanding is that the components of a lightning protection system are installed to remove the strike from the building or structure as safely as possible.
I found it very interesting to read that lightning does not like to and most likely will not follow a conductor around sharp bends.
It is a complex subject, which one day we will all understand more of I am sure.

Pierre

 

[physis]

Re: Lightning Rods:

By Pierre:
lightning does not like to and most likely will not follow a conductor around sharp bends.

That is interesting. I didn't know that!

By Pierre:
The notion that lightning is seeking earth is mostly not true.

I'm not quite understanding this. I know the charges are trying equalize and the ground isn't automaticly the target. I'm under the perhaps mistaken impression that lightning often heads for the earth. Is this a symantics issue or is that just not true, or should I say correct?

[ November 29, 2004, 09:27 PM: Message edited by: physis ]

 

[hurk27]

Re: Lightning Rods:

Sam I think what he's trying to get across is just the fact that lightning does not always go to earth. Now that's a loaded statement as lightning does strike airplanes, clouds, and even kite's thank's to good old Ben F. But unless you plan to have your home floating up in those clouds,(where I think my head is most of the time :roll: ) The only protection we would be concerned with is, cloud to ground, or ground to cloud lightning. Ground = Earth.

 

[pierre]

Re: Lightning Rods:

Again, like most here I am not a lightning expert. What I have read (and hopefully understand ) is that lightning does not seek out earth (ground). One charge seeks out the opposite charge. Earth happens to be one big target for opposite polarity, hence the cloud to earth/ earth to cloud occurrences.

Think of this:
An overhead cloud with the wind current blowing in one direction lining up electrons in one direction (lets say negative charge). The wind is swirling in a different direction on the tree tops, causing the electrons to line up in the other direction(lets say positive direction). This creates a potential of difference and different polarities increasing the chance for lightning to complete it's path and strike say the tree. This is one scenario and can also be played out along the earth's surface, a rooftop, etc...

Pierre

 

[physis]

Re: Lightning Rods:

So we'd be on the same page if I said earth isn't necessarily the target?

 

[bphgravity]

Re: Lightning Rods:

I think the problem lies with the name we use. "Lightning Protection System" is misleading and should probably be called "Charge Redistribution System".

 

[physis]

Re: Lightning Rods:

How bout "Charge Mediation System"?